Papillomaviruses (PVs) cause benign and malignant tumors in the skin and mucosa. Certain types of human papillomaviruses (HPVs) are associated with human cancers, that is, they are associated with malignant conversion. These include malignancies progressing from respiratory papillomas, skin cancers in patients with epidermodysplasia verruciformis, and anogenital carcinomas, specifically cervical cancers. Cancer of the cervix is reported to be the most common cancer in developing countries and the second most common in women worldwide. Papillomaviruses are associated with greater than 90% of all cases of cervical cancers.
Although papillomaviruses (PVs) are implicated in the etiology of cancers, the complete story of their involvement is not yet known. PVs display remarkable species specificity and strong cellular tropism, and produce both benign and malignant tumors in their natural hosts (Howley, 1996; Lancaster et al., 1982; Pfister, 1987). For example, HPVs have a tropism for squamous epithelial cells and infect only surface epithelia of cutaneous and mucous membranes (Broker and Botchan, 1986; zur Hausen and Schneider, 1987). Although PVs are epitheliotropic, they also show remarkable tissue specificity. Papillomas (warts) have been detected based upon their histological characteristics at numerous sites in humans, including the skin, genital tract, respiratory tract, and oral cavity (Rowson and Mahy, 1967). To date over 75 types of HPVs have been defined based upon their sequence homology (Myers et al., 1996).
Malignant conversion occurs only with infection by some types of HPVs. There are low-risk viruses that are only occasionally associated with cancer. The high-risk viruses commonly associated with malignant conversion include those involved in epidermodysplasia verruciformis and a subset of HPV types that infect the anogenital region.
A virus's host range and cellular tropisms are major determinants of whether or not the outcome of infection is malignant conversion. The restrictions on what type of cells support the replication of a given virus may be dictated at a number of early events in viral infection, including attachment, penetration, uncoating, early gene expression and vegetative viral DNA replication (Roizman, 1990). Some studies have reported that PV virus-like particles can attach to and penetrate a wide variety of cells, suggesting that these cells are susceptible to the virus. However, whether these cells are permissive for replication of PVs has not generally been addressed.
Because PVs show a strong tropism for epithelial cells, host range and tissue tropism (permissiveness) would appear to be determined at a point(s) following attachment and penetration. Because PV virus-like particles can attach and penetrate a wide variety of cells, but only a very narrow range of cell types are permissive for HPV, this suggests that tropism is not a function of attachment and penetration, but rather is an event which follows. Steinberg et al. (1989) provided evidence that control over replication is exerted at the transcriptional level. It appears that blocks in permissivity are exhibited at different viral stages in host epithelial cells versus non-host cell types. Further information on this question would be helpful in understanding and counteracting viral effects. To date, there have been only few reports employing infectious HPV virions that address the molecular mechanisms controlling host range and tissue tropism.
Also, little is known about the differentiation-dependent life cycle of HPVs or how to treat/prevent human papillomatosis. For the majority of HPV types there is currently no efficient in vitro assay for infectivity. Therefore, the study of the basic biology of the virus, especially the assessment of the early phases of infection and investigations of the mechanisms by which HPVs show a strict host range restriction and strong tropism for squamous epithelial cells, is severely limited.
In order to define the determinants of cell/tissue tropism and host range, it is desirable to have a PV infectivity assay. For example, HPVs are dependent upon the state of cellular differentiation to complete the viral life cycle, and the viruses do not appear to display lytic properties. Thus, it is not feasible to develop an in vitro infectivity system based upon the final stages of the life cycle (e.g., a plaque assay or an assay for viral particles). Although mouse C127 cells can be used in focus forming assays for BPV1 and BPV2 virus infection, neither HPV from a plantar wart nor cottontail rabbit papillomavirus (CRPV) were able to induce C127 foci (Dvoretzky et aL, 1983). New methods are needed to assess early phase infections.
The E2 gene product is necessary for the replication and episomal maintenance of viral genomes (Del Vecchio et al, 1992; Ustav and Stenlund, 1991). Thus, E2 expression is required soon after infection to replicate the viral genome copy number to the basal number of 50-200 copies per cell (Broker and Botchan, 1986; Lambert, 1991, Ustav and Stenlurid, 1991). E2 is an important viral transcriptional regulator, facilitated by binding as a dimer to the conserved palindromic sequence ACCGN.sub.4 CGGT SEQ ID NO:1, known as an E2 binding site (E2BS) (Androphy et al, 1987; Hawley-Nelson et al., 1988; Moskaluk and Bastia, 1987). In addition, E2 proteins can transactivate heterologous promoters containing several E2BSs located upstream or downstream of the promoters (Spalholz et al., 1988; Thierry et al., 1990). Because the E2 BS sequences are highly conserved among PVs (Myers et al., 1996), a single E2BS assay system potentially could be useful for the analysis of multiple classes of PVs. E2 binding sites are reported to be in different arrangements for cutaneous compared to mucosal lesions. (Garrido-Guerrero et al., 1996).
Nucleotide sequences are reported as probes for the presence and type of papillomavirus (Bouma et al., 1996; Meijer et al., 1994; Herzog et al., 1991). However direct assays that can not only detect the presence of PVs but titer the number of infectious particles, are needed. Comparisons of susceptiblity of different cell types and stages would benefit prevention and therapeutic regimes.